During development of the anterior pituitary, multiple cell fate decisions specify five endocrine cell types. As development proceeds, the identity of each cell emerges marked by the sequential activation of genes specific to that lineage. Two of these cells, the gonadotrope and thyrotrope, produce the glycoprotein hormones luteinizing hormone (LH) and thyroid- stimulating hormone (TSH), respectively. In the previous grant period, we have created five immortalized cell lines which represent the gonadotrope/thyrotrope lineage at specific stages of development. The focus of this renewal application is to investigate the events that determine specific transitions in the differentiation of this cell lineage. In the first Aim, we will investigate cell fate decisions during commitment to the gonadotrope lineage. Expression of the GnRH receptor and the orphan nuclear receptor SF-1 mark the commitment of an alpha subunit-expressing progenitor cell to the gonadotrope lineage and activation of the LHbeta subunit gene marks a further step in maturation. Utilizing the genes activated during this developmental progression, we will seek to identify the key regulators controlling cell fate decisions and differentiation in the gonadotrope lineage. In the second Aim, we will investigate the differentiation of the thyrotrope. The committed thyrotrope expresses alpha subunit, TSHbeta subunit, and the POU-homeodomain protein Pit-1. By comparing regulation of these genes in a mature thyrotrope with progenitor cells expressing only alpha subunit of Pit-1, we will endeavor to define the pathway of thyrotrope differentiation. New technology has made it possible to switch an oncogene on or off in culture or in transgenic mice. In the third Aim, we will develop pituitary progenitor cell models that may differentiate in culture. Such cells will assist in defining precursor/progeny relationships in this lineage and facilitate study of gene expression during pituitary differentiation. Our substantial progress in developing immortal cell lines representing the developmental stages of the gonadotrope and thyrotrope has provided a strong foundation for our proposed studies. We are presented with unique opportunities to study cell fate decisions in the pituitary and to uncover the mechanisms of pituitary- specific gene regulation. Principles derived from the mechanisms important for organogenesis uncovered in this facile developmental system may be of general utility in understanding the overall processes involved in mammalian development.